I was reading about the Hitachi HDPJ52 Front projector which has a dual iris system to increase contrast. Has anyone had any sucess in using an iris system in their projector?

I don't know about an exact iris system like the Hitachi uses, but SamuraiJack made a light engine that pretty much did the same thing... check out his Plog. I think his results are pretty good.

besides that, i can't think of anyone who has. maybe you could be the first?

I think he is talking about an Iris for the projection lens to improve contrast, But this would cut down on the lumen output.

Joe

Yes, Joe is right I'm talking about an iris system to increase contrast. The Hitachi PJ seems to have a butterfly valve infront of the bulb, I wasn't sure where the other iris is. I wish they had internal pictures. I guess the Hitachi automatically adjusts the iris according to the incomming signal? I may try to use an iris in my projector when I build it. I will only have room on the wall for a 60"-65" picture so a little light reduction hopfully won't be much of a problem.

could you explain it something more... I just don´t caugh how an iris could improve contrast.

i've been thinking of that too... i can only imagine that having an iris would just decrease lumen output.

The iris isnt really to block light from the projected cone itself but from the stray light that may hit it produced by diffraction and bouncing. The "off cone" rays will interfere and cause lower levels of contrast. Think of it like radio static. The finer you can tune the channel, the less interference. Thats why I built mine with an iris on the gate itself. Thats basically to address the stray light cause by the wide arc. Im thinking about putting one on the other side of the fresnel array as well. The overall brightness might take a hit with an iris, but the contrast picks up.

Ahhhh! it all makes sense now! you explain it so well...

ok.. so it is something comparable to painting the enclosure with flat black... is it?

Yes, except that the flat black will only absorb rays that come into contact with it. Since we are focusing a lot of light in the direction of the triplet you have a lot of light that is both off axis, and not getting absorbed by the black. The painting of the enclosure to black will help with a lot of the "more off access" stray light. I like both in combination. Quite frankly anything that cuts down on scatter is good. Thats why I was able to run my components without putting them in the box, but even then I still had light wash.

Thanks Allen, I have been looking for a better way to explain it.

So precisely where in the light path between the front fresnel and triplet would one place an iris for best effect? Intuitively, I'd guess the nearer the triplet the better? And are we assuming use of one basically two-dimensional (planar) iris? Am I making sense here? Perhaps I'm not fully understanding the discussion. Or would multiple irises be much better? Or extrapolate that into a bottomless/topless truncated four-sided pyramid with the front fresnel (or LCD in the case of unsplit optics) and the triplet resting at the respective bottom and top of said pyramid. Paint this pyramidal tunnel flat black or line it with something that really absorbs the stray/scattered/junk light. For that matter wouldn't such a pyramid work on the lamp side as well? Hmm . . . maybe the pyramid isn't good but many, many irises lined up along this pyramidal light cone would serve best to ensnare and eliminate stray/scattered light? Kind of bugs me to use the word iris since I always think of CIRCLES and not RECTANGLES. My brother-in-law is on the verge of buying a CNC plasma cutter. Perhaps I'll have him cut me a half dozen rectangular irises to place along the light path and do some research LumenLab style!? Seems the multiple iris thing would indeed perform best - at least in my mind's-eye projector - which, after all, is all that I have at the moment!!!

Come to think of it the brightness loss shouldn't be all that significant from adding a post-fresnel iris . . . should it???

Nope. In this case what we are really talking about is a mask. The iris I use is an adjustable rectangle shape. This would be a static cutout based on the geometry of the LCD. You could then move it back and forth along the path till you get the correct cutoff. I think the return would diminish with each one you placed, but it might make for some seriously good light absorbers.

SJ

EDIT: In my design I have an adjustable iris and a mask. The order it goes is bulb-mask-iris-rear fresnel.

mmm, I don´t caught it yet...

Idealy If we paint the all the triplet side of the box with flat black, there is light only going from the LCD to triplet (all the rest will hit the wood and will be absorbed by the paint). So what is the benefit going for an iris instead of painting? (flat black paint efficieny being bad?)

Whoa nellie. Gotta check back with your L_O_N_G plog but do you really have an iris and a mask (two distinct planes with rectangular cutouts) or are your referring to those "flyers" - things that looks like the blinders horses wear. Yeah, diminishing returns for sure, but two or three roughly equispaced "frontal" irises might get you right near to perfectamundo! I'm going with a vertical so maybe I'll try to stick a couple in - one between the LCD/mirror and one between the mirror/triplet. I can't wait to start blowing all my evenings and weekends once my pro stuff is delivered!

Know what? I just noticed you typed bulb-mask-iris-rear fresnel. I gotta go back through your plog!!! I'm sooooo confused now!

Yeah, the proof IS in the pudding. Still, the idea "sounds sound".

Off to SJ's plog I go . . .

Yep. The holder for the 2 inch slot of shott glass is the mask and the iris is directly on the front of the uniut. It is a 4 direction adjustable. The flyers are so I can cut a 16x9 ratio if I want to.

Start here...Start of Iris construction for square apeture

I had read a while ago about companies utilizing an iris type of system at the projection lens. the iris - or call it a shutter was wired in to the proccessing unit in the projector. the purpose was to have the shutter open when the film had a dark scene but rapidly open/close in the bright scenes. this was for the purpose of adding contrast. I would think one could make something like this with some old camera parts for the shutter and a light sensor to activate it. anyway I think that this was more of what you were thinking.